The immediate effects of the arousal neurotransmitter norepinephrine on the brain include increased alertness and focus, immediate enhancement of short-term mem-ory, dilatated pupils, increased muscle tone, and divergence (outward movement) of the eyes to expand the field of view. The immediate effects of body-based epi-nephrine and brain-based norepiepi-nephrine prepare the organism for the high-level neuromuscular activity required for ensuring survival in the face of threat—the fight/
flight response. These effects promote short-term preparation of the brain for intense alertness, and the neuromuscular and cardiovascular systems for high-level short-term skeletal muscle and cardiac activity and energy expenditure. Activation of the fight/flight response, of course, may be triggered by excitement as well as by threat.
FIGURE 2.2 Hypothalamic/pituitary/adrenal axis. Sensory input signaling stress or threat (see Figure 2.1 ) activates the hypothalamus, triggering release of corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP). These promote release of cortisol from the adrenal medulla. Cortisol inhibits further release of ACTH, modulates the basic noradrenergic arousal response, and mediates the long-term stress adaptation response to stress.
Hypothalamus [CRH, AVP] Stimulate Stimulate
InhibitAnterior Pituitary [ACTH] Adrenal Cortex [Cortisol]
Long-Term Mediation
Modulates, Inhibits HPA Axis
Threat/Stress - Cardiovascular adaptation - Increased arousal, vigilance - Immune suppression - Growth suppression- Decreased vegetative function (Catabolism)
The basic physiological sympathetic nervous system response of the prey in response to threat is mirrored by that of the predator as it prepares for attack. Pre-game jitters, stage fright, sexual arousal, and the thrill of the roller coaster ride all reflect the physical sensations associated with arousal in the face of threat. What separates the experience of the fight/flight response from that of anticipatory excitement, of course, is the meaning of the event to the participant. This piece of information processing takes place in the hippocampus (comparison of new information with past associative memories), and the orbitofrontal cortex (prob-lem solving and planning). 4 Whether the animal or human then assumes the role of predator or prey based on this higher level cognitive process, the end result of epinephrine-based arousal is similar—the initiation of intense muscular activity and exertion. Excitatory neurotransmitters and hormones fuel both the attack of the predator and the flight of the prey, initiating a high-level energy expenditure response in both creatures.
Many species operate both as prey and predator in the pecking order of eating and being eaten throughout the animal kingdom, and mobilize epinephrine at both ends of the spectrum. Adrenaline is an equal opportunity energizer, and whether the weasel is catching a mouse, or being caught by a hawk, the initial physiological effects of both predation and flight are similar in many respects, and contribute to the function of resisting, escaping, or attacking. Very soon, however, the meaning of the arousal sequence assumes an important role in subsequent events involving the brain and hormonal systems. Mason has introduced the role of “psychological”
meaning into the stress response equation, disputing Selye’s avoidance of cogni-tive mediation of the stress response. Under Mason’s concepts, a more complex interaction of cortical/hypothalamic/pituitary circuitry is involved in the stress, or flight response, and allows for a broader range of behavioral response to threat or arousal. Mason maintains that the neurological and hormonal events of preda-tion and defense diverge and lead to quite different behavioral and physiological responses. 6
Other brain neurotransmitter systems also contribute to the effectiveness of the fight/flight response. Norepinephrine, as we have noted, facilitates alerting mecha-nisms and transmission of messages throughout the brain’s central sympathetic pathways. Endorphins, the brain’s pain-modulating neurotransmitters, are also released, accounting for the well-documented increased pain threshold in acute arousal. 7 , 8 The obvious function served by endorphins includes blunting of pain reflexes and inhibition of conscious and unconscious self-protective responses that might interfere with effective survival behavior. For example, with the aid of the analgesic effects of endorphins released with fight/flight arousal, the injured sol-dier is able to complete the physical activity necessary for his survival without the interference of debilitating pain. The wounded prey animal is able to continue its fight/flight behavior without instinctively tending to its wounds.
Release of endorphins as part of arousal also has interesting implications con-cerning brain reward mechanisms in human behavior. Since arousal related to predation or excitement is also linked to endorphin release, it is easy to appre-ciate the reward involved in high-risk recreational activities assoappre-ciated with the
“adrenaline rush,” because this experience also involves the pleasurable reward of endorphins. Risk-taking thrills seem unique to the human species, and it is clear that certain personality traits may predispose to this kind of behavior. Some evi-dence suggests that this trait may be apparent in childhood. At any rate, one does not see weasels jumping into the talons of hawks just to experience the thrill of the wild ride.
Once again, the meaning of the event that triggers the relatively stereotyped initial neurological and chemical contributions to the fight/flight response likely has a marked effect on later similar behavior. Risk taking and the human element related to reward from endorphins in arousal may well contribute to the peculiar tendency for trauma reenactment in PTSD, as explored in Chapter 9 .